A fuel cell converts the chemical energy of fuel into the electric energy by electrochemically reacting fuels in a stack to generate electricity, unlike combustion which generates heat. The fuel cell supplies electric power to a small electric/electronic product, particularly to a portable device as well as industrial, household and vehicular power.
Currently, polymer electrolyte membrane fuel cell or proton exchange membrane fuel cell (PEMFC) having the greatest power density has been used as a power supply source for a vehicle, and thus has been most widely studied, since such fuel cell has a fast start-up time and fast power conversion response time due to its low operating temperature.
The PEMFC includes a membrane electrode assembly (MEA) including the catalyst electrode layers for electrochemical reaction and the solid polymer electrolyte membrane where the catalyst electrode layers are attached to both side thereof and protons move through; a gas diffusion layer (GDL) which uniformly distributes the reaction gas and transfers generated electrical energy; a gasket and a locking unit for maintaining the airtightness and the proper tightening pressure of reaction gases and cooling water; and a bipolar plate where reaction gases and cooling water moves.
When the fuel cell stack is assembled using the above described unit cell components, the combination of the MEA and the GDL is positioned innermost part of the cell, the MEA where the catalyst electrode layers such as an anode and cathode are applied to both sides of the polymer electrolyte membrane for reacting hydrogen and oxygen, and a gas diffusion layer, gasket are laminated sequentially on the outer portion where the anode and the cathode are positioned.
A bipolar plate is positioned outside of the gas diffusion layer, and the bipolar plate supplies the reaction gases such as fuel, hydrogen and oxidant, oxygen or air. In addition, a flow path passing cooling water is formed in the bipolar plate.
Then, a plurality of unit cells are stacked, and then a current collector, an insulating plate and an end plate for supporting the stacked cells are combined in the outermost position, and the fuel cell stack is configured by repeatedly stacking and tightening the unit cells between the end plates.
The unit cells are stacked to obtain the sufficient electric potential for a vehicle power source, and hereafter, stacking unit cells is referred to as a stack. For example, an electric potential generated from one unit cell is about 1.3 V, and a number of cells are stacked in series to generate the power for driving a vehicle.
Meanwhile, in the fuel cell vehicle, a portion of the output voltage of the stack is used to diagnose the state of a fuel cell.
In the related arts, a method for diagnosing a state of a fuel cell has been developed. After an alternating current such as sine wave is applied to the fuel cell stack, the output voltage of the fuel cell stack is measured, the measured voltage is frequency converted and the harmonic component is detected, and then the state of the fuel cell stack is diagnosed by detecting the linearity or non-linearity of the fuel cell stack based on the detected harmonic component.
However, because the harmonic component is detected by using the output voltage of the fuel cell stack, when the variation of the output voltage is substantial, the deviation of the harmonic component may increase due to the mix of the harmonic component by the voltage variation, and as consequence, detection accuracy for the harmonic component is reduced.
The description provided above as a related art of the present invention is just merely for helping understanding of the background of the present invention and should not be construed as being included in the related art known by those skilled in the art.